I. Field of the Invention
The invention is directed to a process for producing seamless steel pipes or flat products (strips, sheets) for pipes or vessels for the conveyance, transporting or processing of hydrocarbons. Corrosive conditions exist when CO.sub.2 and water and possibly small proportions of H.sub.2 S are present in the media to be transported or processed.
II. Discussion of the Prior Art
For production or working of hydrocarbons under corrosive conditions, pipes made of low-alloy steels with passive corrosion protection (inhibition) or high-alloy corrosion-resistant steels are normally used to meet the strict requirements respecting resistance to corrosion, in particular also resistance to stress crack corrosion. A suitable steel is known from DE 26 16 599 C2, for example. Due to the high proportion of expensive alloying elements (e.g., 22% Cr, 5% Ni, 3% Mo), pipes and vessels made from such steels are extremely cost-intensive when used for the aforementioned purpose. These relatively high-strength compound or duplex steels usually have a low carbon content and can therefore be welded easily.
Steels containing 0.18 to 0.22 % carbon and 12.5 to 14% chromium (AISI 420) are also known for use in oil fields. This material has very good resistance to corrosion in a moist CO.sub.2 environment. Since it is practically impossible to weld pipes produced from this material under construction site conditions, the pipes are connected exclusively by screw connections. Therefore, pipes produced from such steels are used only as conveying pipes, but not as line pipes. If traces of H.sub.2 S are also contained in the hydrocarbons to be conveyed through the pipes, damage may occur as a result of stress crack corrosion, since this work material has only a comparatively low resistance to this type of corrosion.
Further, weldable 13-percent chromium steels are also known for producing steel pipes. An example is AISI 410 (work material No. 1.4006) which contains 0.08 to 0.12% carbon, a maximum of 1.0% manganese and 12.0 to 14.0% chromium. The weldability of this steel is ensured by the low carbon content. However, heat treatment of rolled products produced from this steel is often problematic as it frequently results in an inhomogeneous joint which is responsible for the very poor resistance of these steels to stress crack corrosion in the presence of H.sub.2 S. For this reason, this work material which is considered resistant to rust and acids is used for pump pipes, heat exchangers and the like, but not for conveying hydrocarbons. It is used for accoutrements or fittings in the region of the bore shaft head only as cast or forged products. Its limited resistance to corrosion has been sufficiently documented in written reports relating to cases of damage.
Finally, a steel is known from JP 57-5849 for the production of seamless steel pipes having the following composition:
______________________________________ max. 0.015% C 0.10-0.80% Si 0.10-2.00% Mn max. 0.025% P max. 0.010% S 11.0-17.0% Cr 0.10-3.00% Ni max. 0.015% N 0.01-0.05% Nb 0.01-0.10% Al ______________________________________
remainder iron and usual impurities. PA1 remainder iron and usual impurities,